def sum_normal(plist):
    sum = 0;
    for i in plist:
        sum = sum + i
    t = sum /len(plist)
    return t

def sum_bad(plist):
    sum = 0;
    for i in plist:
        sum = sum + i
    t = sum /len(plist)
    plist[0] = 10
    return t

def sum_good(plist):
    plist = plist[:]
    sum = 0;
    for i in plist:
        sum = sum + i
    t = sum /len(plist)
    plist[0] = 20
    return t

list1 = [1,2,3]

#print(sum_normal(list1))

#print(sum_bad(list1))
#print(list1[0])


#print(sum_good(list1))
#print(list1[0])

list2 = list1[:]  
print(sum_bad(list2))
print(list1[0])